Injection molding and transfer molding processes are commonly used in the manufacture of plastic articles. In both of these processes, the operation begins with the loading of a solid resin into a molding machine. The resin is heated and melted under pressure and the hot liquid resin is transferred from a reservoir to the mold cavities. When the resin cools, the part is ejected from The mold. In transfer molding, a reactive resin is used and a thermoplastic resin is used with injection molding. Molding presses for these processes consist of a preheater, a hydraulic power unit, a platen, a multi-cavity mold, and a control unit for controlling the process. The transfer molding resin is in the form of a pellet and is placed into the preheater. After the resin has reached the desired temperature, a plunger compresses the pellet and maintains the forcing pressure for ten to fifty seconds, depending upon the mold temperature and the type of resin. Once the resin has softened to a workable viscosity range, the plunger transfers the resin into the mold cavity via runners and gates. This transfer is completed within a few seconds. Once the resin has been forced into the mold cavity, the resin reacts or cures to form a thermoset material, the cavity is then opened, and the solid part is ejected.
The injection molding process is similar to transfer molding except that a continuous supply of resin is provided to a heated feed screw that heats and pressurizes the resin, converting it into a molten state. The feed screw rapidly transfers the molten resin under pressure into the mold cavity where the resin quickly cools and solidifies. The mold cavity is then opened and the finished part is ejected.
In both these processes, the rheology of the molten resin is critical to the outcome of the molded part. If the resin is not heated properly prior to injection, the mold cavity will not fill because the resin viscosity is too high. Conversely, if the resin is overheated prior to injection, degradation of the thermoplastic polymer or premature curing of the reactive resin will occur.
Conventional methods of monitoring the rheology of materials are cumbersome and require several types of information. For example, the flat-plate orifice method requires knowledge of the relative orifice sizes, the fluid velocity past the orifice, and the pressure of the system on both sides of the orifice. Other methods of monitoring the material rheology are performed in an indirect manner. Micromet Instruments, Inc. of Cambridge, Mass. produces a sensor that measures the ionic viscosity of a material by measuring the relative amount of ionic species in a material, which gives inferential information about the chemical structure of a polymer. As a thermoset polymer reacts, the mobility of ionic species decreases with the subsequent increase in solution viscosity. This type of measurement does not directly measure the rheological property of a material, but provides inferential information about the properties. Other test methods require that samples of finished parts be removed from the mold and submitted to an off-line test where melt flow rate and other physical properties of the molded plastic may be determined. One example of an off-line measurement system is the Tinius-Olsen extrusion plastometer, made by Testing Machine Company of Willow Grove, Pa. Once this is known, molding parameters such as temperature, time, and pressure are then closely controlled in an attempt to achieve uniformity. It is clear that this type of off-line measurement of the resin properties does not yield real-time information and can only be performed on a batch basis, requiring the destruction of at least part of the molded product. A method of determining the viscosity and rheology of the molten plastic during the molding operation would be extremely beneficial to controlling and monitoring the quality of a continuous molding operation.